I'm not sure if I set something incorrectly, but my signal menu looks like this:

https://i.imgur.com/XW94zij.png

Settings:

https://i.imgur.com/G7vocyw.png

Ah I see, I set recipe category width to 7 and now it matches the width, except there's a small spacing where the scrollbar is:

https://i.imgur.com/M4JE1TV.png

The entity preview is also not centered/stretched properly:

https://i.imgur.com/PPW8XZC.png

Thanks for the reports. It might take some time to get all this fixed since I'm not a modder myself. So it will rely on the person who created these changes in the first place (or others) to step in and help. Thank you for your patience!

Sounds good, thanks!

Ah I see, I set recipe category width to 7 and now it matches the width, except there's a small spacing where the scrollbar is:

Set number of slots lower, or number of categories higher and there wont be any extra spacing.

The entity preview is also not centered/stretched properly

Not the mod problem. It just changes number of slots\categories per row and nothing more. How it behaves with higher than default values is up to factorio. If it actually bothers you - write a bugreport on factorio forums.

Ah I see, I set recipe category width to 7 and now it matches the width, except there's a small spacing where the scrollbar is:

https://i.imgur.com/M4JE1TV.png

The entity preview is also not centered/stretched properly:

https://i.imgur.com/PPW8XZC.png

Eulenberg has addressed your concerns as much as possible with this PR. It sounds like some of the things you're observing are just artifacts of having two levers to pull, and if you pull them in different directions, you'll see some weird stuff. And the entity preview isn't capable of stretching that way apparently.
https://github.com/Krydax/Custom-Production-UI-Fixed/issues/3#issuecomment-3046446513

Thank you :)